1. Field of the Invention
The present invention relates to an improved, reliable solar cell module which excels in weatherability and in resistance to light degradation and which continuously exhibits a desirable photoelectric conversion efficiency without being deteriorated even upon repeated use over a long period of time under severe environmental conditions of high temperature and high humidity. More particularly, the present invention relates to an improved solar cell module comprising a photovoltaic element enclosed by a filler resin and a surface protective member disposed to cover said photovoltaic element, said surface protective member being composed of a specific fluororesin containing an ultraviolet absorber (hereinafter referred to as a UV absorber) dispersed therein, wherein the photovoltaic element is tightly sealed by means of the aforesaid specific surface protective member with an improved adhesion to the filler resin. The solar cell module excels especially in light transmission and weatherability. Further, the solar cell module excels in resistance to light degradation, moisture resistance, heat resistance and abrasion resistance, and it continuously exhibits a desirable photoelectric conversion efficiency without being deteriorated even upon repeated use over a long period of time under severe environmental conditions of high temperature and high humidity.
2. Related Background Art
In recent years, heating of the earth because of the so-called greenhouse effect due to an increase of atmospheric CO.sub.2 has been predicted. In view of this, there is an increased demand for a means of power generation capable of providing clean energy without causing CO.sub.2 building.
In order to meet such demand, there have proposed various solar cells which can supply electric power without causing such a problem as above mentioned and are expected to be a future power generation source.
Such a solar cell includes single crystal silicon solar cells, polycrystal silicon solar cells, amorphous silicon solar cells (including microcrystal silicon solar cells), copper indium selenide solar cells, and compound semiconductor solar cells. Of these solar cells, various studies have been made on so-called thin film crystal silicon solar cells, compound semiconductor solar cells, and amorphous silicon solar cells since their semiconductor active layer can be relatively easily formed in a large area and in a desired form and they therefore can be easily produced at a relatively low production cost.
Particularly, thin film amorphous solar cells, specifically, amorphous silicon solar cells, comprising a metal substrate, an amorphous semiconductor active layer disposed on said substrate, and a transparent and conductive layer disposed on said semiconductor active layer have been evaluated as being the most advantageous among the conventional solar cells because their semiconductor active layer comprised of amorphous silicon (hereinafter referred to as a-Si) can be easily formed in a large area and in a desired form on a relatively inexpensive substrate with a low production cost and they are light and excel in shock resistance and flexibility, and in addition, they can be designed into a solar cell module in a desired configuration which can be used as a power generation source.
Now, in the case of an amorphous silicon solar cell having a semiconductor active layer comprising, for example, an a-Si thin film disposed on a glass plate as a substrate, light is impinged through the substrate side, and because of this, the glass plate can be designed to serve as a protective member. However, in the case of the aforementioned solar cell having the a-Si semiconductor active layer disposed on the metal substrate, because the metal substrate does not permit incident light to transmit therethrough, light is impinged through the side opposite the metal substrate, and therefore, it is necessary to dispose an appropriate transparent protective member on the side through which light is impinged such that it protects the solar cell element. In the conventional manner of doing this, a transparent fluorine-containing polymer film having a good weatherability is used as the surface protective member and a transparent thermoplastic resin is used as a filler under the transparent fluorine-containing polymer film.
In fact, the fluorine-containing polymer film is often used in the above described manner, since it is advantageous in that it is satisfactory in terms of weatherability and water-repellency and serves to diminish a deterioration in the photoelectric conversion efficiency of the solar cell element caused due to a reduction in the transmittance of the surface protective member which can occur when the protective member is yellowed or clouded as a result of the protective member having been deteriorated.
As for the thermoplastic resin used as the filler in combination with the fluorine-containing polymer film, it is also often used since it is relatively inexpensive and suitable for protecting the solar cell element.
Now, description will be made of a conventional solar cell module with reference to FIG. 4.
FIG. 4 is a schematic cross-sectional view of an example of a conventional solar cell module. In FIG. 4, reference numeral 401 indicates a photovoltaic element (or a solar cell), reference numeral 402 a transparent thermoplastic resin as a filler, reference numeral 403 a transparent surface protective member comprising a fluorine-containing polymer thin film, and reference numeral 404 an insulating member.
In this solar cell module, the filler disposed on the rear side of the photovoltaic element 401 is comprised of the same thermoplastic resin as that disposed on the side of the photovoltaic element through which light is impinged.
Specific examples of the fluorine-containing polymer thin film used as the transparent surface protective member 403 are fluororesin films such as ETFE (ethylene-tetrafluoroethylene copolymer) film, PVF (polyvinyl fluoride) film, PVDF (polyvinylidene fluoride) film, and the like. Specific examples of the transparent thermoplastic resin used as the filler 402 are EVA (ethylene-vinyl acetate copolymer), butyral resin, and the like.
The insulating member 404 is disposed in order to reinforce the solar cell module while adding an appropriate rigidity thereto. The insulating member 404 is usually comprised of an organic resin film such as nylon film, aluminum foil sandwiched with TEDLAR (trademark name), or the like.
The thermoplastic resin 402 serves as an adhesive between the photovoltaic element 401 and the surface protective member 403 (that is, the fluororesin film) and also as an adhesive between the photovoltaic element and the insulating member 404. In addition to this, the thermoplastic resin 402 also serves as a filler for preventing the photovoltaic element from being externally damaged and from suffering from external shocks.
However, the conventional solar cell module thus constituted has such problems as will be described in the following, because the transparent thermoplastic resin is insufficient in weatherability. That is, when the solar cell module is continuously exposed to sunlight in the outdoors over a long period of time (for example, over 20 years), the thermoplastic resin used as the filler is liable to gel due to continuous irradiation of ultraviolet rays, resulting in its being clouded or it is liable to yellow due to an increase in the number of conjugated double bonds in the chemical structure of the resin as a result of continuous irradiation of ultraviolet rays. The occurrence of such cloudiness or yellowing makes the thermoplastic resin used as the filler to be poor in light transmittance, resulting in reducing the photoelectric conversion efficiency of the solar cell module. Hence, the thermoplastic resin as the filler is insufficient not only in terms of weatherability but also in terms of durability upon repeated use in the outdoors over a long period of time (for example, over 20 years). And these problems will become significant when the solar cell module is placed on the roof of a building or is integrated with the roof and it is continuously used in severe outdoor atmospheres of high temperature and high humidity.
Herein, as for the solar cell module in which the filler is constituted by EVA, it is known that when the solar cell module is continuously used at a temperature of 80.degree. C. or above, the filler is liable to remarkably yellow.
In addition, the deterioration of the transparent thermoplastic resin as the filler causes not only the foregoing problem of reducing its light remittance but also another problem of extinguishing its rubber elasticity and/or reducing its adhesion. In the case where the filler is poor in rubber elasticity, there is a fear that a stress which occurs when the photovoltaic element and/or the electric connection terminals are expanded or shrunk due to a sudden change in the environmental temperature or when the solar cell module is deformed due to an external force applied thereto is not sufficiently absorbed and damages the photovoltaic element or causes a separation between the photovoltaic element and the filler. Further, in the case where the filler is poor in adhesion, there is a fear that a separation is liable to occur between the filler and the photovoltaic element or between the filler and the surface protective member.
Hence, there has not been provided a desirable organic material which has a highly improved weatherability, is highly transparent, and is capable of desirably attaining the requirement desired for the surface coating member of a solar cell module.